Optoelectronic display

ABSTRACT

An optoelectronic display comprises a substrate  32  of semiconducting material and an array of organic light emitting diode (OLED) pixels arranged on the substrate, which comprises an active circuit for controlling the light emitted from each pixel. Each pixel comprises at least one layer of organic light emitting material  46 , and an (at least semi-) transparent electrode  48  in contact with the organic layer on a side thereof remote from the substrate, the electrode comprising an electrically conducting polymer  50.

BACKGROUND TO THE INVENTION

The present invention relates to an optoelectronic display. Organiclight emitting diodes (OLEDs) comprise certain organic materials whichare known to emit light under electrical stimulation. The materials caneither be small molecules or polymer materials (in polymer lightemitting diodes, PLEDs). These materials require different processes forpractical manufacture into display devices. Small molecule materials aredeposited onto a substrate by vapour deposition whilst polymers are castonto a substrate from a solution by spin-coating, printing, doctorblading or a reel to reel process. In a typical polymer LED, a polymerlayer is deposited, by spin coating, onto indium tin oxide (ITO) coatedglass. This is followed by heat treatment to drive off residual solventand a reflective metal electrode is then evaporated onto the top surfaceof the polymer layer. The ITO, which is transparent, forms the otherelectrode and the polymer emits light through the ITO coated glass whena voltage is applied between the electrodes. Current and voltage controlof the light emission is known.

Both types of materials and processes have been used to fabricate arrayson a number of different transparent and non-transparent surfaces.Methods known in the art for creating full colour displays includeink-jet printing of polymer solutions and vapour deposition of smallmolecule materials. Other known methods include the use of monochromedisplays fitted with individual absorptive filters or colour changingmedia filters. Whilst both materials appear compatible with photo-resisttechnology, in practice the processing has reduced the efficiency andlifetime of the devices to unacceptable levels. High resolution colourand monochrome displays have been demonstrated for small molecules bydepositing them into microcavities. In EP-0,774,787, a full colour OLEDarray is fabricated on a CMOS substrate by this method. The drivers forthe diode array are formed in the substrate. The diodes are addressed bya passive matrix of conductive strips. For high resolution displaysactive matrix address schemes are preferable as they are more efficient.

Several different types of flat panel displays have been fabricated withactive matrix address schemes. For instance, various types of liquidcrystal display have been fabricated on crystalline silicon (LCOS) andother silicon materials such as polysilicon on glass. The siliconmaterial provides the active matrix drive circuitry as well as thesubstrate. Similarly, a vacuum fluorescent display has been fabricatedon crystalline silicon.

The manufacture of arrays of OLEDs on non-transparent substrates such asCMOS or bi-CMOS is hindered by the need to fabricate an (at least semi-)transparent electrode on top of the organic layers to allow lightemission and viewing. Deposition of indium tin oxide directly onto theorganic layers can cause unacceptable deterioration in the deviceperformance. Another consideration is the need to carefully select thechoice of metal electrode material directly in contact with thesubstrate so that it is fully compatible with microelectronicmanufacturing equipment.

SUMMARY OF THE INVENTION

According to the present invention there is provided an optoelectronicdisplay comprising a substrate of semiconducting material and an arrayof organic light emitting diode (OLED) pixels arranged on the substrate,wherein the substrate comprises an active circuit for illuminating eachpixel, and each pixel comprises at least one layer of organic lightemitting material, and an (at least semi-) transparent electrode incontact with the organic layer on a side thereof remote from thesubstrate, the electrode comprising an electrically conducting polymer.

Preferably, the substrate is of crystalline silicon and the surface ofthe substrate may be polished or smoothed to produce a flat surfaceprior to the deposition of elctrode, or organic, materials of each OLED.Alternatively a non-planarised silicon substrate can be coated with alayer of conducting polymer to provide a smooth, flat surface prior todeposition of further materials.

In one embodiment, each OLED pixel comprises a metal electrode incontact with the substrate. Depending on the relative work functions ofthe metal and transparent electrodes, either may serve as the anode withthe other constituting the cathode.

The metal electrode may consist of a plurality of metal layers, forexample a higher work function metal such as aluminium deposited on thesubstrate and a lower work function metal such as calcium deposited onthe higher work function metal. In another example, a further layer ofconducting polymer lies on top of a stable metal such as aluminium.Preferably, the electrode also acts as a mirror behind each pixel and iseither deposited on, or sunk into, the planarised surface of thesubstrate. However, there may alternatively be a light absorbing blacklayer adjacent to the substrate.

In still another embodiment, selective regions of a bottom conductingpolymer layer are made non-conducting by exposure to a suitable aqueoussolution allowing formation of arrays of conducting pixel pads whichserve as the bottom contacts of the pixel electrodes.

The organic light emitting material is preferably a polymer but mayalternatively be a monomer or a transition metal chelate. Apart from thelight emitting material, organic layers in the pixel elements mayinclude an electron transport material layer, a hole transport materiallayer, a protective cap material layer and a conducting polymer materiallayer.

As well as a conducting polymer, the (at least semi-) transparentelectrode may comprise further layers, e.g. of indium tin oxide (ITO) orother transparent or semi-transparent metal oxides or low or high workfunction metals, or conducting epoxy resin, deposited onto the organiclayer furthest from the substrate. Alternatively, a glass or plasticsheet, coated with ITO, conducting polymer, or at least one of thelayers that constitute the (at least semi-) transparent electrode, maybe bonded to said furthest layer or another layer of this electrode, tocomplete the electrode and serve as a barrier to the ingress of oxygenand water . The viewing surface of the display may be completed byencapsulation with a further layer of polymer or glass.

The preferred conducting polymer is poly(ethlyendioxythiophene), sold byBayer AG under the trade mark PEDOT. Other molecularly alteredpoly(thiophenes) are also conducting and could be used, as could theemaraldine salt form of polyaniline. To improve the adherence of PEDOTto certain smooth substrates a polymer blend with a non-conductingpolymer, preferably poly (vinyl alcohol) (PVA), can be made. Forexample, a 9 wt % solution of PVA with PEDOT in a 10(PVA) :6 volumeratio can be used. A wide range of molecular weights of PVA can be usedwithout much difference in the resultant film or its conductivity.

High work function metals that could be used include tungsten, nickel,cobalt, platinum, palladium and their alloys, and possibly niobium,selenium, gold, chromium, tantalum, hafnium, technetium and theiralloys.

The brightness of light emitted from each pixel is preferablycontrollable in an analogue manner by adjusting the voltage or currentapplied by the matrix circuitry or by inputting a digital signal whichis converted to an analogue signal in each pixel circuit. The substratepreferably also provides data drivers, data converters and scan driversfor processing information to address the array of pixels so as tocreate images.

In one embodiment, each pixel is controlled by a switch comprising avoltage controlled element and a variable resistance element, both ofwhich are conveniently formed by metal-oxide-semiconductor field effecttransistors (MOSFETs). In an alternative embodiment, also preferablycomprising MOSFET switches, the apparent intensity of light output froma pixel is controlled by varying the mark/space ratio of the duty cyclefor which the LED is switched on, preferably by means of an analoguevoltage value. This relies on the fact that for duty cycles less thanabout 40 ms, the eye perceives only the average brightness of the pixelduring its entire duty cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be more readily understood,reference will now be made, by way of example only, to the accompanyingdrawings, in which:

FIG. 1 is a schematic circuit diagram of an active matrix array of pixelcircuits according to an embodiment of the invention;

FIG. 2 shows a generic pixel circuit;

FIG. 3 shows a specific pixel circuit implementing the generic pixelcircuit of FIG. 2;

FIG. 4 is a schematic cross section of a single pixel of a planarisedsubstrate according to an embodiment of the invention (not showing thepolymer LED);

FIG. 5 is a schematic cross section of an alternative substrate, showingthe deposited polymer LED, and

FIG. 6 is a schematic, fragmentary side view of an array of polymerLEDs.

FIG. 7 is a schematic view of a color display unit according to thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a rectangular array of pixel circuits 2 and circuitry foraddressing them. The intensity of light to be emitted by a polymer orother organic LED is controlled by varying the current flowing throughit. This is done by applying analogue signals, provided by column lines4 and set up by a column (data arrange) circuit 6, to the pixel circuits2 on a row-by-row basis. The required row 8 is briefly selected by a rowselect circuit 10 and then deselected. During the selection time slot,the data from the column lines 4 flows into the pixel of the selectedrow. When deselected, each pixel circuit 2 is isolated from, its columnline 4 and stores the data that was input. Another set of data isassembled on the column lines 4 and another row 8 is selected. Rows maybe selected sequentially or in any desired order.

FIG. 2 shows a generic form of the pixel circuit 2 indicating itsoperating principle. The signal on row 8 operates a voltage-controlledswitch 12 to close the switch during the brief selection period andconnect column bus line 4 with a node in the circuit whose signalcontrols an electronically programmable variable resistance 14. Thus,data flows from the column bus 4 into the pixel circuit 2, current froma global power source 16 flowing through a LED 18 at a value set by theresistance 14. The intensity of light output by the LED is controlled inturn by that current. Different light intensities can be set veryeffectively in the different pixels in this manner.

When the switch 12 opens, the data is maintained with the pixel circuituntil different data is presented when the switch closes at the nextselection of the row 8.

FIG. 3 shows how the circuit of FIG. 2 is implemented in a specificembodiment of the invention using MOS (metal-oxide-semiconductor)transistors. The voltage controlled switch is formed by a firsttransistor M1, the gate of which is connected to the row 8. The variableresistance is formed by the channel of a second transistor M2, whichcapacitatively stores charge at its gate to vary the channel resistancedepending on the analogue value of the voltage generated by the storedcharge. This controls the current in the LED 18 and its light output.

This is a simple but effective active pixel circuit. If it is found tosuffer from pixel-to-pixel performance variations (for example due tovariations in transistor threshold voltage) additional active elementscan be used. Thus, for example, current mode line drivers 20 (instead ofvoltage mode line drivers) can be used to drive the column lines 4 shownin FIG. 1. Adding the additional elements to the column lines as shownis preferable to adding them to the pixel circuits, since if the arrayis square and contains n² pixel circuits, only n additional sets ofelements (e.g. 128) will be required rather than n² (e.g. 16,384).

Once the active matrix circuitry has been fabricated in thesemiconductor substrate, for example using CMOS technology, the surfaceof the substrate is planarised. This planarisation either takes place aspart of the manufacturing process of the integrated circuit or as asubsequent customising step.

As shown in FIG. 4, the planarisation is effected by depositing adielectric 30, for example a polymeric material, on the surface of thesubstrate 32. A conducting polymer that can be patterned to create areasof insulation can be used instead for this purpose. A metalmirror/electrode 34, which may be of aluminium, for connecting the LEDto the appropriate point in the circuit, is then deposited, theconnection to the circuit being established by a metallic conducting via36. Metallised portions of the CMOS circuit are designated 38.

FIG. 5 shows an alternative arrangement in which the electrode/mirror 34is sunk into the dielectric surface, i.e. full planarisation isachieved.

FIG. 5 also shows one way in which the display construction can becompleted. Appropriate layers 40 of the LED (e.g. polymer or otherorganic light emitting substance, conducting polymer and the like) aredeposited and the display is sealed by coating with a glass plate 42coated on its inner surface with a transparent conducting layer 43 whichmay in particular be of ITO, conducting polymer or both.

FIG. 6 shows an alternative display construction including a particularexample of the deposited layers. On the substrate 32 there aredeposited, in turn, the planarised aluminium electrode/mirror 34,optionally an electron or hole transport layer 44, a light emittingpolymer 46, and a transparent electrode 48. The transparent electrodemay for example consist of a thin layer of high work function metal 49,of a thickness to be adequately transparent, a layer of conductingpolymer 50 and a layer of ITO 51. An encapsulation layer/barrier 52which seals all of the LEDs of the array, including their sides,completes this example of the display construction, three pixels ofwhich are shown in FIG. 6.

In manufacturing the display shown in FIG. 6, the flat metal mirrors 34are applied to the surface of the substrate 32 (preferably a CMOS orbi-CMOS backplane) so as to cover most of the area of each pixel withminimal gaps between the mirrors. Chemical Mechanical Polishing may beused to enhance the global and local planarisation.

The layers of polymer and related materials can be deposited by anautomated technique using equipment currently used for applyingphoto-resists used for the patterning of integrated circuit layers. Thisgives precise control and a highly uniform thickness for each layer.Alternatively, the polymer layers could be ink-jet printed. Rare earthchelates can be vacuum deposited.

The encapsulation layer 52 is applied after making the connections tothe transparent electrode in each pixel. Encapsulation, and alsopossibly the assembly of the pixel, are carried out in clean, dryconditions under a partial vacuum, or a suitable inert or controlledatmosphere.

The display of the invention may be monochromatic. However, monomericand polymeric substances are now available which will emit either red,green, blue or white light and can therefore be used to form OLEDsemitting those colours. Thus, a full colour display 60 can be formed byarranging three individual backplanes, 62, 64 and 66, each emitting adifferent primary monochrome colour, on different sides of an opticalsystem 68, from another side of which a combined colour image 70 can beviewed. See FIG. 7. Alternatively, polymers or other organic substancesemitting different colours can be fabricated so that adjacent diodepixels in groups of three neighboring pixels produce red, green and bluelight. In a further alternative, field sequential colour filters can befitted to a white light emitting display.

Optical systems can also be used for increasing the apparent size of thedisplayed image, since the physical size of display is limited by thesize of the silicon substrate. For example, the image can be projectedon to a screen.

The display of the invention is robust, the organic LEDs being wellprotected, but has simplified manufacture and encapsulation. The powergenerated as heat should be manageable but could be decreased byreducing the current or voltage used to drive each LED. If currentrouting problems arise, multiple power supply bond pads can be used onthe silicon chip.

1. An optoelectronic display comprising a non-planarized substrate ofsemiconducting material and an array of organic light emitting diode(OLED) pixels arranged on the substrate, wherein the substrate comprisesan active circuit for controlling the light emitted from each pixel, andeach pixel comprises at least one layer of organic light emittingmaterial, and a light-permeable electrode in contact with the organiclayer on a side thereof remote from the substrate, the electrodecomprising an electrically conducting polymer provided as a coating onone of a glass sheet and a plastics sheet which is bonded to the organiclayer furthest from the substrate.
 2. A display according to claim 1,wherein the substrate is of crystalline silicon.
 3. A display accordingto claim 1, further comprising a metal electrode in contact with thesubstrate which also serves as a mirror behind the pixel.
 4. A displayaccording to claim 1, further comprising a light absorbing black layeradjacent to the substrate.
 5. A display according to claim 1, whereinthe light-permeable electrode includes a layer of indium tin oxide(ITO).
 6. A display according to claim 5, wherein said ITO layer is alsoprovided as a coating on said sheet.
 7. A display according to claim 1,wherein the light-permeable electrode includes a layer of low workfunction metal.
 8. A display according to claim 1, wherein thelight-permeable electrode includes a layer of high work function metal.9. A display according to claim 1, wherein the light-permeable electrodeincludes a layer of conducting epoxy based resin.
 10. A displayaccording to claim 1, wherein each pixel includes a bottom electrodecomprising a layer of conducting polymer.
 11. A display according toclaim 1, wherein each pixel includes a bottom electrode comprising alayer of metal oxide.
 12. A display according to claim 1, wherein eachpixel includes an organic electron transport layer in contact with thelayer of light emitting material.
 13. A display according to claim 1,wherein each pixel includes an organic hole transport layer in contactwith the layer of light emitting material.
 14. A display according toclaim 1, wherein the conducting polymer is deposited from a polymerblend solution including at least one non-conducting polymer.
 15. Adisplay according to claim 1, further comprising a transparent, oxygen-and water-impermeable, encapsulating outer layer.
 16. A displayaccording to claim 1, wherein the light emitting material is monomeric.17. A display according to claim 1, wherein the light emitting materialis polymeric.
 18. A display according to claim 1, wherein each OLEDcomprises a transition metal chelate.
 19. A display according to claim1, wherein the apparent brightness of light emitted from each pixel iscontrollable in an analogue manner.
 20. A display according to claim 19,wherein an analogue signal varies the mark/space ratio of the duty cyclefor which the OLED of each pixel is switched on.
 21. A display accordingto claim 20, wherein each pixel circuit comprises a variable resistanceelement for varying the current through the OLED and hence its lightintensity output.
 22. A display according to claim 21, wherein saidvariable resistance element comprises the channel of ametal-oxide-semiconductor field effect transistor (MOSFET).
 23. Adisplay according to claim 21, wherein each pixel circuit includes avoltage-controlled switch for connecting a data signal to said variableresistance element so as to adjust its resistance.
 24. A displayaccording to claim 23, wherein said switch comprises a transistor.
 25. Adisplay according to claim 1, further comprising repeated groups ofred-, blue- and green-emitting pixels for forming a color image.
 26. Adisplay according to claim 1, wherein the display is arranged to emitwhite light and is fitted with field sequential color filters forcreating color images.
 27. A color display unit comprising: threeoptoelectronic displays, each optoelectronic display having anon-planarized substrate of semiconducting material and an array oforganic light emitting diode (OLED) pixels arranged on the substrate,the substrate providing an active circuit for controlling the lightemitted from each pixel, and each pixel having at least one layer oforganic light emitting material and a light-permeable electrode incontact with the organic layer on a side thereof remote from thesubstrate, the electrode being an electrically conducting polymerprovided as a coating on one of a glass sheet and plastic sheet which isbonded to the organic layer furthest from the substrate, and eachoptoelectronic display displaying an image in a different primarymonochromatic color, and an optical system for combining the threeimages into a color image.
 28. An optoelectronic display comprising asubstrate of semiconducting material and an array of organic lightemitting diode pixels arranged on the substrate, wherein the substratecomprises an active circuit for controlling the light emitted from eachpixel, and each pixel comprises at least one layer of organic lightemitting material, and a light-permeable electrode in contact with theorganic layer on a side thereof remote from the substrate, the electrodecomprising an electrically conducting polymer provided as a coating onone of a glass sheet and plastics sheet which is bonded to the organiclayer furthest from the substrate.
 29. A display according to claim 28,wherein a layer of indium tin oxide is also provided as a coating onsaid sheet.
 30. A display according to claim 28, wherein thelight-permeable electrode comprises a layer of epoxy resin.